When you duplicate a property/column of type single select, it seems to work fine.
But when you then try to change the type of the source property, it will empty out the values of the destination (duplicate) property. It seems to me that after duplication, both properties are a reference (pointer) to the same data. When you change a value of a row in the destination (duplicate), that value will remain. Doing so seems to break the link (desired behavior during duplication).